Fans are used in carburizing and carbonitriding furnaces to circulate a controlled gas atmosphere through the work in an effort to obtain uniform heating of the work and to enhance the transfer of carbon and nitrogen from the atmosphere to the surface of the work. Their effectiveness depends on the flow rate and the degree of uniformity of flow that can be achieved at the surface of the work. Prior art furnaces have utilized fans mounted in the top of the furnace, in the side walls, or in the furnace floor. Such prior designs are subject to the disadvantage of utilizing a controllable flow through the work from only one side of the fan, either the suction or the pressure side. In addition, prior designs have not been able to achieve substantially uniform and controlled flow throughout the length and width of a processing zone.
According to the present disclosure, a continuous carburizing furnace has aligned heating, carburizing and diffusion zones, and longitudinally spaced piers on the floor of the furnace in each of the zones. Work supporting trays are conveyed along suitable conveyer means supported on the piers. Each zone is provided with a fan for circulating the controlled gas atmosphere within that zone through the work. The fan is mounted in the lower portion of a side wall of the furnace in such position that both its inlet and its outlet are at a level below the level of the work trays, with the inlet and outlet longitudinally spaced apart and separated by at least one of the piers in such zone so that the gas atmosphere is pulled downwardly through the work at one or more pier positions and is forced upwardly through the work in the remaining pier positions of the processing zone. The refractory material is designed in relation to the fan inlet and outlet in such a way as to provide a controllable and substantially uniform flow on both the suction and pressure sides of the fan throughout the length and width of the processing zone.